1. Field of the Invention
The present invention relates to a semiconductor laser diode, in particular, a laser diode providing a plurality of optical guiding layers in a side of an n-type cladding layer.
2. Related Prior Art
A United States Patent, U.S. Pat. No. 5,923,688, has disclosed a semiconductor laser diode (hereinafter denoted as LD) that includes an active layer, and a pair of separate confinement hetero-structure (SCH) layers sandwiching the active layer therebetween. The SCH layers, configured with a multiple layers more than two layers, have a band gap energy gradually increasing as a part from the active layer. Such SCH layers suppress an inner loss due to the increase of the carriers within the active layer and degradation of the quantum efficiency due to the overflowing of the carriers from the active layer. Thus, the LD disclosed in the '688 patent enables to operate in higher ambient temperatures with substantially superior efficiency.
A conventional LD generally provides a graded index structure, which is often called as GRIN structure, and a stepped-FRIN structure to effectively confine light and electrons in the active layer and layers adjacent to the active layers. The stepped-GRIN structure includes a plurality of guiding layers to enhance the optical confinement efficiency, where layers closer to the active layer have a composition to show the higher refractive index, which is equivalent to a configuration where the band gap energy of each guiding layers becomes smaller as the layer approaches the active layer. Especially, a type of conventional LD that provides an active layer including AlGaInAs or AlInAs often provides the first guiding layer, which is apart from the active layer, with a band gap wavelength shorter than 1.1 μm. However, such guiding layers, put between the active layer and the cladding layer, are generally doped to lower the differential resistance of the device.